Introduction: As the primary negative regulator of oncogenic AKT pathway signaling, PTEN functions as one of the most important tumor suppressors in glioma biology. Contrary to numerous findings that imply PTEN expression should be retained in a significant portion of glioblastomas multiforme (GBMs) (~60%), our own immunohistochemical data indicates that this number is considerably lower (10-20%), implying additional, nongenomic mechanisms for PTEN silencing. In the present study, we investigated the role of PTEN promoter methylation and microRNA PTEN silencing and associated GBM oncogenesis.
Methods: We studied a collection of 80 GBM tumor samples from patients undergoing craniotomy at a major academic cancer center. Methylation-specific PCR of PTEN promoter was performed by DNA bisulfite conversion of methylated cytosines to uracils, followed by PCR using primers complementary either to the original PTEN promoter sequence (where mC->mC) or to the mutated promoter sequence (where C->T). microRNA profiling was done using RT-PCR with primers for miR-19a, 19b, 21, 26a, 106b, and 214 (previously identified in low-grade gliomas). Quantitative PCR was performed relative to endogenous RNU6B and normal brain.
Results: Our immunocytochemistry and DNA mutation data indicated that the loss of PTEN protein can happen even in absence of gene mutation or deletion. Both promoter methylation and upregulation of microRNAs was found to be associated with silencing of PTEN expression in glioblastoma.
Conclusions: Our results suggest that promoter methylation and microRNA overexpression may be two distinct mechanisms responsible for epigenetic silencing of PTEN in brain tumors. Understanding these epigenetic mechanisms can lead to better prognostic tools and new drug targets for GBM.
Patient Care: It will improve our ability to make accurate prognosis, and identify potential drug targets.
Learning Objectives: By the conclusion of this session, participants should be able to understand the role of DNA methylation and microRNA in GBM oncogenesis.